Would you say it is high performing students attracted to the school or high performing students created at the school? I know that education at these schools is indeed very high quality and wholistic in terms of scope (indeed due to resources available to them) and that it isn't just 99.95's but a large proportion (10% for Scotch College) achieving above a 99.5.
I'd say a combination of both. When high performing students are attracted to an elite school, it usually means they want to be surrounded by other high achieving students and an all round strong cohort for a number of reasons such as SAC moderation. In order for a student to consolidate their knowledge at these schools, they would require the foundations of the subject otherwise regurgitating their 'elite resources' would be pointless.
At the end of the day, their material isn't really the key factor to the school's prosperity; instead it's the compilation of top tier students where they are relatively at the same level due to their strong build up in their early years. If material was a major factor, then my school would've been amongst those schools you named as both my methods and physics teachers exchange resources with teachers from Haileybury and Nossal. When completing their worksheets, only 2-3 people actually do reasonably well, whilst the rest suffer. Sadly, it was the 2-3 people who were actually brought up with a solid base in both mathematics and science.
To top it off, the maths/physics teacher at my school is the owner of iTute, who makes literally all the exams himself (Genius of a man) - for those that doesn't know, iTute is known to make ridiculously hard exams. The tests that he make aren't bad considering you have been taught with a strong foundation because they will really test your knowledge about maths/physics!